As is well known, a variety of photographically useful compounds are used in photographic materials. Included therein is a group of compounds which are converted to different compounds through photographic processing steps, such as development, fixation, and washing, to release a photographically useful compound that can be used to complete photographic processing. A number of compounds belonging to this group are known and have been put into practical use. The photographically useful compounds include, for example, image forming dyes, filter dyes, sensitizing dyes, development inhibitors, antifoggants, development accelerators, silver halide solvents, redox compounds, hardening agents, discoloration inhibitors, and many others. Some of them release such a photographically useful compound upon being reduced during processing steps. For example, in the case of an image forming dye-releasing compound, it is reduced to release a dye in areas where silver development does not occur while it is not reduced and no dye is released in areas where silver development proceeds. As a result, there is obtained a reverse of a developed silver image. Likewise, a development inhibitor-releasing compound or an anti-foggant-releasing compound releases a development inhibitor to effectively inhibit silver fog in areas where silver development proceeds a little while a development inhibitor is not released in areas where silver development actively proceeds or is necessary thereby to eliminate adverse effects, such as development retardation.
It is required that the above-described compound capable of releasing a photographically useful group upon being reduced should be combined with a reducing agent and, in many cases, the both compounds are incorporated in the same layer or layers adjacent to each other so that a redox reaction may take place between them in an appropriate stage of processing to reduce the photographically useful group-releasing compound.
However, because they exist in close proximity with each other, they unavoidably undergo gradual reaction with each other during long-term preservation. In particular, if activity is increased in an attempt to make the rate or quantity of the release during processing higher, preservation stability is further deteriorated. Thus, preservation stability and activity are counteractive. It has been, therefore, keenly demanded to develop a technique which achieves both high activity during processing and preservation stability.